Field of the Invention
The present invention relates to laser systems, and more particularly to laser illuminator systems.
Description of Related Art
Laser illumination systems project a beam of collimated light across an area. Often this is done to enable observation of a laser illuminated area in a particularly useful but limited range of wavelengths. This can be done for example to illuminate an area with light that is not visible to people but that can be detected electronically such as by illuminating an area with near infrared light or short wave infrared light, or this can be done to sense objects in a scene that may be fluoresce when illuminated when exposed to specific wavelengths of light.
One problem with observing laser illuminated areas is while a laser illumination may be generally uniform, non-specular surfaces in the illuminated area may reflect the coherent light from the laser such that interference patterns arise when the light is observed by a person or electronic imager. The interference creates areas that appear to be unnaturally bright and areas that appear to be unnaturally dark creating a high noise component in the reflected light observed in an area. The interference effect is known as speckle.
Speckle is visually distracting and can make it difficult for both human observers and automatic vision systems to detect contrast patterns in the illuminated areas.
Saloma, et al. in a paper entitled “Speckle reduction by wavelength and space diversity using a semiconductor laser”, published in Applied Optics, Vol. 29, No. 6, (Optical Society of America 1990) describe a speckle reduction system that uses modulation of a laser to create additional longitudinal modes, with each mode having a different laser frequency. In operation, mode hopping is used and a grating is used to introduce a shift in a position of a point of a source of the illumination as a function of the change in frequency during the mode hopping. The change in position reduces the extent of the speckle contrast when averaged over time.
Trisnadi, in a paper entitled “Speckle contrast reduction in laser projection displays”, published in Projection Displays VIII, Ming H. Wu, Editor Proceedings of SPIE Vol. 4657, (SPIE 2002) describes generally speckle reduction strategies as methods for averaging N independent speckle configurations with the spatial and temporal resolution of a detector and identifies three different mechanism for speckle reduction: wavelength diversity which requires a laser with a sufficiently large range of wavelengths to reduce speckle, polarization diversity which requires emission of laser light having two different polarizations and angle diversity which requires shifting the point of illumination. Trisnadi proposes a combination of polarization and angle diversity to achieve speckle reduction. In Trisnadi, angle diversity is accomplished using a moving diffuser.
Geske et al. U.S. Pat. No. 8,743,923 describe the use of a multi-wavelength VCSEL array to reduce speckle using wavelength diversity. In this embodiment, the VCSEL array has a plurality of laser emitters each with a different wavelength creating a laser emitter having a broad enough bandwidth to reduce the speckle effects.
What is need in the speckle reduction art is a solid state laser device that does not require the grating and extended optical path of Saloma, that does not require moving parts like the moving diffuser of Trisnadi and that does not require the complexity and cost of a VSCEL array.